Supercritical isobutane regeneration of an ultrastable Y-zeolite (USY) alkylation catalyst was examined in a continuous, automated reaction/regeneration system. Two feeds were studied: a synthetic isobutane/2-butene blend and a commercial refinery isoparaffin/olefin blend. Synthetic feed experiments showed that high levels of butene conversion were maintained for a time on stream of more than 200 h, and that product quality and catalyst maintenance was relatively stable over the course of the experiment, using a 3 h reaction/3 h regeneration cycle. Thirty five reaction/regeneration cycles were conducted over 210 h using the synthetic feed. Over this period, butene conversion did not drop below 92% until the end of the experiment. The average butene conversion declined 0.051% per run, and the average C-8 and trimethylpentane (TMP) composition decreased only 0.082% and 0.06%, respectively, per run. Catalyst activity maintenance was lower when the commercial feed was used. High levels of alkene conversion were maintained for 78 and 192 h, using a 3 h reaction/3 h regeneration cycle and a 2 h reaction/2 h regeneration cycle, respectively. Using a 3 h reaction/3 h regeneration cycle, alkene conversion remained at or above 92% for 13 reaction/regeneration cycles, corresponding to 78 h of semicontinuous operation. Using a 2 h reaction/2 h regeneration cycle, the alkene conversion did not drop below 92% until the end of 48 reaction/regeneration cycles, corresponding to 192 h of operation. The 2 h reaction/2 h regeneration cycle limited the decline in alkene conversion to 0.035% per run and limited the decline in C-8 and TMP composition to 0.040% and 0.042%, respectively, per run.